1. Field of the Invention
The present invention relates to a pattern evaluation method, a method of manufacturing a semiconductor device, a program and a pattern evaluation apparatus.
2. Related Art
By way of example, a conventional pattern evaluation will be described in the context of evaluation of a micropattern on a semiconductor. However, the present invention is not limited to the field of semiconductor pattern, but provides a new technique for measuring pattern dimensions and shape in various industries, including for example evaluation of a glass substrate such as a photomask.
For a semiconductor device manufacturing process, it is important to evaluate dimensions of a micropattern formed by a photolithography process, etching process, etc. Generally, CD-SEM (Critical Dimension Scanning Electron Microscope) is used to measure and evaluate pattern dimensions (e.g. the line width of a line pattern, the bore diameter of a hole pattern, etc.). CD-SEM measures pattern dimensions or the like by capturing secondary electrons at a detector, which is generated by irradiation of a primary electron beam into a pattern, and performing a predetermined image processing for the SEM image of the pattern that is obtained as the signal intensity of the secondary electrons. This will be described below in the context of the determination of the line width of a line pattern.
First, the scanning line direction of an electron beam is determined to be perpendicular to the line pattern to obtain the SEM image. Then a portion is cut from the SEM image according to a region of interest (ROI) described by a measurement recipe or specified by an operator, and for example, stored in an image memory in a computer running an image processing. Assuming that X direction is the scanning line direction of the electron beam in the ROI, an intensity value for each line pixel is detected to make a line profile, from which required edge points (in this case, two edge points at left-hand and right-hand sides) are detected by for example, the threshold method. Taking the distance between these edge points (the difference of X coordinates) as the width of the line pixel in the line pattern, the line pattern width is calculated for each line pixel. The average value of the resulting values over ROI's length direction (Y direction) is output as the line pattern width. Also, in order to remove a variation of computation values due to image noise, a value can be determined by averaging for example, over three line pixels including upper and lower line pixel, then the resulting value can be considered as the width of the line pixel. Further, the value which is obtained from averaging the value over ROI's length direction can be finally output as the width of the line pattern.
In this way, in the case of measuring pattern dimensions, following two steps are required as a image process: first, the procedure of detecting edge points using an edge point detection algorithm, then the pattern dimensions is calculated from coordinates of the detected edge points by a CD measurement algorithm meeting with the object of the measurement. In these steps, the maximum slope method or the like has been proposed as the edge points detection algorithm, in addition to the threshold method and the line approximation method (e.g. see Japanese Laid Open 09(1997)-184714).
As the CD measurement algorithm, in addition to simple methods such as using the average distance in the direction of X axis between edge points, or calculating the maximum and minimum distance between edge points as the maximum and minimum line width respectively in the case of a line pattern whose width is not constant, there are also more complex methods such as those proposed in Japanese Laid Open 2000-171230.
In addition to these methods of measuring a pattern directly from edge points, there is also another method in which a profile of a pattern is represented by some means, and measurement of the pattern profile is used in place of the pattern measurement. For example, in the case of measuring a hole pattern, a technique has been proposed in which an approximation circle is calculated from the resulting edge points, and the diameter of the calculated approximation circle is used in place of the diameter of the hole pattern. Further, as with the case of an OPC profile of a reticule in order to evaluate a more complex pattern, it has been proposed that edge points is connected in string so that the edge points is represented as a polygon curve.
However, in the method in which edge points is connected in string so that the edge points is represented as a polygon curve, since an actual pattern profile is approximated as an unnatural profile line, an inaccurate measurement has been obtained especially if the number of edge points is small. Therefore, among the values representing characteristics of a pattern shape (hereinafter referred to as “characteristic values”), especially the characteristic values associated with pattern profile such as a rounding of a turn corner portion or the roughness of a pattern for example could not measure accurately. On the other hand, in the case of large number of edge points, handling with a computer was difficult due to the increase of CPU (Central Processing Unit) load in the processing computer.